Electrolytic production of aluminium using the Hall-Héroult electrolytic process is well known. In the Hall-Héroult process aluminium is produced from Al2O3 dissolved in an electrolytic bath of molten cryolite and AlF3 at a temperature of about 960° C. using carbon anodes. Aluminium ions are reduced to aluminium at the cathode while oxygen is combined with carbon at the anode to form CO2 gas.
In this process about half a kilogram of carbon is consumed for each kilogram of produced aluminium. Carbon anodes must therefore routinely be replaced. The CO2 gas produced at the anode is considered to be a green house gas, and is an undesired by product of the process.
Efforts have been made to provide inert and dimensional stable anodes for use in the electrolytic production of aluminium. Most of the research has concentrated in oxide-based ceramic anodes and cermet anodes. However, these efforts have so far not been commercialised for production of aluminium.
U.S. Pat. No. 5,254,232 describes an anode for use in molten electrolyses of metals such as aluminium. The anode comprises an alloy of the product metal such as aluminium and a more noble metal upon which is formed an oxide of the product metal as a protective layer. For electrolysis of aluminium the protective layer consists of Al2O3. All surfaces of the anode intended to be in contact with the electrolyte in the cell have this protective layer. The anode of U.S. Pat. No. 5,254,232 suffers from the disadvantages that the protective layer may be dissolved, particularly if the content of alumina in the electrolyte becomes low. Thus one has to operate the electrolytic cell with an electrolyte which is saturated with alumina. This can cause operational problems such as accumulation of undissolved alumina at the bottom of the electrolytic cell and will further provide problems in controlling the cell operation. If the protective layer of the anode is dissolved, the alloy of which the anode is made can be consumed resulting in failure of the anode.
U.S. Pat. No. 6,083,362 describes an anode for use in electrolytic production of aluminium, where the anode comprises a substrate made from a metal alloy defining a cavity where the substrate comprises an interior wall and an exterior surface where the substrate is capable of diffusing aluminium from the cavity to the exterior surface to provide a film covering portions of the exterior surface and mean for replenishing the film. The means for replenishing the film on the exterior surface of the substrate is a molten salt containing aluminium and an anion selected from the group consisting of a fluoride, a carbonate, a chloride, an oxide and combination of these.
The anode described in U.S. Pat. No. 6,083,362 has some disadvantages. The aluminium in the fluid salt composition is depleted as the aluminium diffuses through the substrate to the exterior surface. Thus, the concentration of aluminium in the salt varies and additional aluminium must be supplied to the molten salt in the cavity from time to time during the use of the anode to maintain the concentration of aluminium in the fluid molten salt bath in the cavity. Such periodic additions of aluminium to the salt is impractical in a commercial operation where the electrolytic cell is closed. Furthermore, variation in the aluminium content of the salt will cause variation in the composition and thickness of the protective layer and have a deleterious effect on the operation of the electrolytic cell. Finally as the aluminium content in the salt bath decreases it will be difficult to maintain a homogeneous concentration of aluminium in the salt bath. This may cause a too low aluminium activity locally in the salt bath resulting in a too low diffusion rate of aluminium through the metal alloy which may cause permanent changes locally in the metal alloy making it impossible to maintain the protective layer locally on the outside of metal alloy. It should be appreciated that no stirring device or other means for maintaining a homogeneous aluminium concentration in the salt bath is described in U.S. Pat. No. 6,083,362.
There is a need for commercial process for the production of aluminium using a dimensionally stable anode where the protective layer is stable.